itself? It must also agree in kind, place, and line of direction, with the generating vibration. A vibratory motion, which recurs t times in a second, cannot beget a diminutive one that recurs ½ t, or 2 t times; nor one originally impressed on the region of the brain corresponding to the auditory nerves, beget diminutive vibrations in the region corresponding to the optic nerves; and so of the rest. The line of direction must likewise be the same in the original and derivative vibrations. It remains therefore, that each simple idea of sensation be attended by diminutive vibrations of the same kind, place, and line of direction, with the original vibrations attending the sensation itself: or, in the words of the proposition, that sensory vibrations, by being frequently repeated, beget a disposition to diminutive vibrations corresponding to themselves respectively. We may add, that the vibratory nature of the motion which attends ideas, may be inferred from the continuance of some ideas, visible ones for instance, in the fancy for a few moments.
This proof of the present proposition from the foregoing appears to be incontestable, admitting the fourth: however, it will much establish and illustrate the doctrines of vibrations and association, to deduce it directly, if we can, from the nature of vibratory motions, and of an animal body; and not only from the relation between sensations and ideas. Let us see, therefore, what progress we can make in such an attempt.
First, then, if we admit vibrations of the medullary particles at all, we must conceive, that some take place in the fœtus in utero, both on account of the warmth in which it lies, and of the pulsation of those considerable arteries, which pass through the medullary substance, and which consequently must compress and agitate it upon every contraction of the heart. And these vibrations are probably either uniform in kind and degree, if we consider short spaces of time; or, if long ones, increase in a slow uniform manner, and that in degree only, as the fœtus in utero increases in bulk and strength. They are also probably the same in all the different regions of the medullary substance. Let these vibrations be called the natural vibrations.
Secondly, As soon as the child is born, external objects act upon it violently, and excite vibrations in the medullary substance, which differ from the natural ones, and from each other, in degree, kind, place, and line of direction. We may also conceive that each region of the medullary substance has such a texture as to receive, with the greatest facility, the several specific vibrations, which the objects corresponding respectively to these regions, i.e. to their nerves, are most disposed to excite. Let these vibrations be, for the present, called preternatural ones, in contradistinction to those which we just now called natural ones.
Thirdly, Representing now the natural vibrations by N, and the preternatural ones, from various objects, by A, B, C, &c. let us suppose the first object to impress the vibrations A, and then
